ZHAODONG, China—The Huiji Hotpot restaurant is a local favorite here, where diners boil meat and vegetables in cauldrons of broth—comfort food to gird against the subzero winter in this far northern farming community.

A couple of years ago customers started to quiz manager Wu Xiaofeng: Did his restaurant use oil made with genetically modified soybeans in its kitchen? He hung a sign next to Huiji Hotpot’s cash register, pledging no.

“We felt it was better just to tell them not to worry,” Mr. Wu said.

Such worries aren’t going away soon.

The opposition to genetically altered food and grains in China has been brought to the forefront by China National Chemical Corp.’s $43 billion deal to buy Swiss agro-giant Syngenta AG, a leading producer of genetically engineered seeds. The ChemChina deal would be by far China’s biggest-ever foreign acquisition.

While China doesn’t currently allow planting of such seeds for grains like soybeans, many in the agriculture business expect that to gradually change once the Syngenta acquisition clears regulatory hurdles, expected later this year.

Any changes could face resistance from local farmers and other Chinese. “All we know is that it’s not natural,” said Li Shubin, who grows corn on his family’s 3-acre plot in Changfu village, near Zhaodong. His farmhouse is heated from an oven that burns dried cobs from the field. “There could be problems with the food’s safety, so if that’s the case we wouldn’t dare use it.”

Fear of genetically modified grains stems in part from wide distrust of China’s food industry, where scandals killed or sickened thousands. In one of the worst, tainted milk and baby formula sickened nearly 300,000 children, and killed six, in 2008.

The U.S. government, the National Academies of Sciences, Engineering and Medicine, and even some Chinese leaders say GMO crops are safe. Such products have become common in the U.S. and other countries.

Proponents say the high-tech seeds boost farm yields—a priority for the government as it looks to feed a billion-plus population. Industry executives say they’re needed as part of broad reforms to boost harvests and avoid more imports.